Lithium-ion secondary batteries are well known as examples of nonaqueous electrolyte secondary batteries. Lithium-ion secondary batteries are employed in smart phones, tablets, notebook computers and other mobile devices because they have superior energy density, output density, charge-discharge cycle characteristics and the like in comparison with other secondary batteries such as lead storage batteries, and they have contributed to reduction in size and weight and high performance of such devices. In terms of input-output characteristics, charging times and the like, however, they have not yet reached the level of performance required for secondary batteries for use in electrical vehicles and hybrid vehicles (vehicle-mounted secondary batteries). Therefore, research is being conducted to improve the charge-discharge characteristics at high current densities (high-rate characteristics) with the aim of increasing output and reducing time for charging nonaqueous electrolyte secondary batteries. Also, since high durability is also required for vehicle-mounted applications, compatibility with cycle characteristics is required. In particular, techniques are in demand for maintaining advanced cycle characteristics because the cycle characteristics are often reduced in designs that increase the energy density directly connected to cruising distance per charge by increasing, for instance, a thickness of the electrode mixture layer or using a high-capacity or high-voltage active material.
Nonaqueous electrolyte secondary batteries are also required to have excellent durability (cycle characteristics).
Nonaqueous electrolyte secondary batteries are composed of a pair of electrodes disposed with a separator in between and a nonaqueous electrolyte solution. Each electrode is formed of a collector and a mixture layer formed on a surface of the collector, and the mixture layer is formed by, for instance, coating and drying an electrode mixture layer composition (slurry) containing an active material and a binder and the like on the collector.
When high-rate charging/discharging is performed, the active material swells and contracts due to rapid occlusion and release of lithium ions. In order for the battery to exhibit excellent durability even under such conditions, a binder with strong binding ability is required in order to prevent breakdown of the electrode mixture layer, peeling from the collector and other deterioration accompanying charge-discharge cycling.
Meanwhile, in recent years, aqueous electrode mixture layer compositions have also been in increased demand for reasons such as environmental protection and cost reduction. In the context of lithium-ion secondary batteries, aqueous binders using styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) are being used in electrode mixture layer compositions for negative electrodes that use carbon materials such as graphite as the active material. However, further improvements are needed to accommodate the advanced high-rate characteristics and cycle characteristics required for vehicle-mounted applications. Meanwhile, solvent-based binders of polyvinylidene fluoride (PVDF) and the like using organic solvents such as N-methyl-2-pyrrolidone (NMP) are preferred for positive electrodes of lithium-ion secondary batteries, and no aqueous binder has been proposed that fulfills the requirements discussed above.
Active materials such as graphite and hard carbon (HC) and other carbon-based materials including conductive aids such as Ketjen black (KC) and acetylene black (AB) are often used as components of lithium-ion secondary batteries. In general, these carbon-based materials have poor wettability by aqueous media, so as to obtain a uniform electrode mixture layer composition with excellent dispersion stability, an aqueous binder having an excellent dispersion stabilizing effect on these carbon-based materials is desired. When coated on a collector (metal foil), a mixture layer composition with excellent dispersion stability can form a highly smooth electrode mixture layer without defects such as spots and streaks. Moreover, an electrode mixture layer with superior conductivity is obtained when the active material, conductive aid and the like are uniformly dispersed.
The electrode manufacturing process includes steps such as rolling, rerolling, cutting and winding. When, for instance, the electrode mixture layer becomes detached from the collector in any of these steps, productivity (yield) declines due to contamination of the production line and production of defective products and the like. A binder that has strong binding ability and does not cause detachment of the mixture layer is therefore desired.
Under these circumstances, several aqueous binders applicable to lithium-ion secondary battery electrodes have been proposed.
Patent Literature 1 discloses an acrylic acid polymer crosslinked with a polyalkenyl ether as a binder for forming a negative electrode coating of a lithium-ion secondary battery. Patent Literature 2 describes obtaining an excellent capacity retention rate without breakdown of the electrode structure even using an active material containing silicon by using a polymer comprising polyacrylic acid crosslinked with a specific crosslinking agent as a binder. Patent Literature 3 discloses an aqueous secondary battery electrode binder, containing a water-soluble polymer with a specific aqueous solution viscosity comprising a structural unit derived from an ethylenically unsaturated carboxylic acid salt monomer and a structural unit derived from an ethylenically unsaturated carboxylic acid ester monomer.